This removes the reference to a demo on getpinry.com, at least until there is a working demo there.

+ Also add some description on upgrade-how-to

+ Allow overwrite in local_settings.py (the original implemetation
  will cause an KeyError)

Feature：use example file instead of docker-compose.yml / add local settings support

Feature:  Upgrade django to 1.11 LTS to fix webp saving error

New version of django fix the RuntimeError while saving wepb file.

The patch will not be applied to Django 1.8

Ref: https://www.djangoproject.com/download/#supported-versions

Ref: https://github.com/django/django/pull/10331

Feature: Use page title as default description

Feature/referer field

fix #111 auto file delete/original size preview

Also add css to restrict original-image size

For django-images, the AUTO-DELETE mark is required to remove files
automatically.

Fix tags being serialized as stringified map objects

This commit fixes an issue where the `tags` key of pins in the API response would be serialized as the string `"<map object at 0x...>". This issue would cause a `TypeError: pins.objects[i].tags.sort is not a function` on the frontend and prevent the page from loading.

Fix Timestrap => Pinry typo

Add Travis CI test